Democratic Sens. Charles E. Schumer of New York, Patty Murray of Washington, Ron Wyden of Oregon, Jeanne Shaheen of New Hampshire and Christopher S. Murphy of Connecticut will hold a news conference at 12:30 p.m. Thursday to call for the amendments.

On Wednesday, as details from the mass shooting were still emerging, Reid was laying the groundwork for at least a narrow debate over gun control.

In reaction to last week's shooting at a Planned Parenthood clinic, the Nevada Democrat filed an amendment to the GOP budget reconciliation package that would bar gun possession by individuals guilty of breaking a law against obstructing access to reproductive health services through the use of force.

Specifically, the Reid amendment would apply to those convicted of a misdemeanor violation of the Freedom of Access to Clinic Entrances Act. Three people were killed last week by a gunman at a clinic in Colorado Springs, Colo.

Multiple Senate Democrats told Roll Call before the Wednesday shooting in California that the caucus had already been discussing whether to push for firearms-related votes on the reconciliation measure. Merely filing an amendment does not mean it will necessarily get even a procedural vote.

"Gun violence has become a cancer on this nation,'' Reid said in a statement after Wednesday's shooting. "We are better than this. Too often, we are turning on our televisions to scenes of horror like those we are witnessing today. This madness must stop. There is no excuse for inaction. We cannot allow ourselves to grow numb to gun violence or to feel powerless in the face of it. We can, and must, make common sense gun reforms that can keep weapons out of the hands of dangerous criminals and the mentally ill. The American people are crying out for us to act. If we do not act, this distressing reality will not change."

Murphy said he knew of discussions about gun provisions in the reconciliation process, but seemed to think the debate would focus on Obamacare and reproductive health.

He was among the first lawmakers to issue a statement in response to the events in California.

"As we await the facts from the chaos on the ground, I can only pray that America’s leaders will do something — anything — that prevents more communities from knowing this sorrow. Congress' number one responsibility is to keep our constituents safe, and not a single senator or member of Congress can go back to their state this weekend and claim that they are doing their job," Murphy said.

Reid's amendment to the federal criminal code is not related to the budgetary objectives of the reconciliation process, which allows the underlying bill to roll back the Affordable Care Act and defund Planned Parenthood to pass in the Senate with a simple majority and without a Democrat-led filibuster.

Republicans continue to argue that the best way to prevent mass shootings is to focus on mental health. Following the October shooting in Oregon, Majority Whip John Cornyn, R-Texas, told Roll Call his bill would make it easier to compel treatment and encourage states to participate in the National Instant Criminal Background Check System.

Republican Sen. Cory Gardner represents Colorado, where three people were killed in last week's shooting at a Planned Parenthood clinic and 12 people were killed and 70 injured in a 2012 shooting at an Aurora movie theater. He called the shooting "tragic," and said, "we can't be afraid to talk about how we have to reduce violence in this country."

He, too, said mental health legislation, rather than gun control, is the relevant issue. “I haven’t seen the details of what’s happened, but it doesn’t matter because it’s tragic and something that is an unacceptable action by clearly some very vile, evil actors. I hope that we can address this through mental health issues and again, I think that’s the direction we have to go."